Computational studies of stress and structure development resulting from the coalescence of metallic islands

Abstract

Thin film component properties are critical design elements in almost all industries. These films are particularly important in the performance of micro- and nano-electromechanical systems (MEMS and NEMS). Residual stress in thin film components is often treated as an unavoidable side effect of processing steps and the degree of residual stress can drastically affect the performance and properties of the final product. While high levels of residual stress are often detrimental to performance, control of the stress and stress gradients can also be used to enhance performance and even generate new capabilities. The work presented in this thesis examines the role of island coalescence in the development of structure and stress in thin films. The primary methods of investigation are molecular dynamics (MD) and finite element analysis (FEA). The semi-empirical MD calculations show that coalescence is a very rapid process for unconstrained spheres and for hemispheres allowed to slide on a frictionless substrate. Particle rotations are commonly observed during the coalescence calculations. The extent of neck formation between 2 particles is consistent with continuum models even down to length scales which would normally be outside the range in which the models might be expected to be applicable. The MD calculations also show that internal island defects may be induced by the island coalescence process, but only for a particular range of island sizes. We present an energetic model for the existence of such a size range and have located experimental evidence in the literature for such defects. Our FEA work extends an earlier study on the effects of contact angle on island coalescence. Our FEA study of islands with greater than 90 degree contact angle coalescence shows that neck formation occurs very similarly to the free sphere coalescence case. We conclude that MD and FEA calculations are useful tools in analyzing the island coalescence process and can provide mechanistic insight beyond what is available from the more general continuum models.